Circuit boards, comprising a conducting material on the surface of an insulating material, are well-known. One method of making such circuit boards includes providing a rigid substrate with a ground plane and an insulating dielectric layer, and selectively applying conductive material to the insulating layer in the form of a circuit. An electrical connection between the ground plane and the circuit can be made, for example, by drilling a hole through the insulating layer and providing an electrical contact between the ground plane and the conductive material.
The circuitry can comprise various elements including, for example, (1) electrically conductive pathways, e.g. electrically conductive printed circuits formed by thick film technology, and (2) circuitry components such as (a) discrete electronic components, e.g. integrated circuits, resistors, capacitors, etc., and (b) devices employed to interconnect the conductive pathways and the discrete electronic components, e.g. leadless ceramic chip carriers.
Many interconnection devices used in the past had a compliant lead connecting the component to a printed circuit. Because of the compliancy of the lead, thermal expansion was not a critical factor because differences in thermal expansion between the substrate and the device could be handled by the flexing of the compliant lead. More recently, it has become desirable to employ leadless carriers, such as leadless ceramic chip carriers made of alumina a beryllia.
Because the leadless chip carriers are typically directly soldered to the circuit board, there is little or no flexibility to provide for differential thermal expansion. Therefore, as the circuit board expands and contracts with heating and cooling, the solder joints which connect the leadless ceramic chip carrier to the conductive pathways will be placed under stress due to the thermal expansion rate difference between the chip carrier and the substrate. The stress can cause the solder joints to weaken and crack. Additionally, the thermal expansion differential can cause the various layers which make up the circuit board to separate. These problems are exacerbated by dense circuitry which not only generates greater heat but, because of the fineness of the circuit elements, is more susceptible to damage from differences between the linear coefficients of thermal expansion of the various circuit board materials.
As used herein, the term "linear coefficient of thermal expansion" will be understood to mean the ratio of the change in length of an object per .degree. C. change in temperature to the length of the object at 0.degree. C. The value of the coefficient varies with temperature. Therefore, unless otherwise indicated, the values listed herein will be calculated at 25.degree. C. and will be expressed in terms of units of expansion per million units per .degree. C. (ppm/.degree. C.).
Another problem typically encountered with prior printed circuits is heat dissipation. In the past, rigid substrates comprising ceramic materials or other materials which have low heat conduction rates have been employed. As a result, generated heat cannot readily dissipate. Therefore, it would be advantageous to have a substrate with a high thermal conductivity in order to avoid the build-up of heat.
In connection with both the thermal expansion and thermal conductivity problems of previous substrates, the problems become more acute as more and more electronic components are placed on circuit boards. The greater density of electronic components, typically creates a greater amount of heat during operation.
Previous approaches, as described, e.g., in U.S. Pat. No. 4,679,122 by Belke, Jr. et al., issued July 7, 1987; U.S. Pat. No. 3,873,756 by Gall, et al., issued Mar. 25, 1975; U.S. Pat. No. 3,514,538 by Chadwick, et al., issued May 26, 1970; and U.S. Pat. No. 3,334,395 by Cook, et al., issued Aug. 8, 1967, have recognized that superior thermal conductivity can be realized by employing metallic substrates, e.g. aluminum substrates. However, these references do not address the problems resulting from differences in thermal expansion between the circuit components and the substrate. Additionally, the substrates disclosed in these patents include layers of organic materials which cannot withstand high temperatures, such as those employed in thick film processes.
U.S. Pat. No. 3,165,672 by Gellert, issued Jan. 12, 1965, discloses that there is a disparity in coefficients of thermal expansion between metallic circuitry and plastic substrate materials and that plastic substrate materials are undesirable because of their typically poor thermal conductivity, which results in inferior heat dissipation. In order to overcome these problems with plastic substrates, Gellert discloses a substrate comprising a metal core, preferably aluminum, coated with a dielectric material. Circuits are formed on the dielectric layer by an acid etching process. However, in order to connect electrical components to the circuits, holes are drilled through the metal substrate. The holes are filled with an epoxy material. Epoxy materials typically melt, vaporize, oxidize or otherwise degrade at temperatures greater than about 400.degree. C., making them unsuitable for use in connection with high-temperature fabrication processes, such as thick film technologies. The sintering or firing temperatures employed in a thick film process typically reach 900.degree. C. to 1000.degree. C.
U.S. Pat. No. 4,700,273 by Kaufman, issued Oct. 13, 1987, discloses a method for accounting for the thermal expansion of a silicon chip by interposing a layer of molybdenum between a ceramic substrate and the silicon chip. However, the ceramic substrate itself is a poor thermal conductor and therefore heat cannot readily dissipate.
U.S. Pat. No. 4,496,793 by Hanson et al., issued Jan. 29, 1985, recognizes the problems encountered when leadless carriers are mounted on a substrate having a coefficient of thermal expansion different than that of the leadless carriers. Hanson et al. disclose matching the coefficient of thermal expansion of a substrate to that of a carrier by producing a laminate substrate with stabilizing layers of a metal having a coefficient of thermal expansion less than that of the laminate structure as a whole. The laminate structure includes bonding sheets which are formed of resin impregnated glasscloth. This type of approach is not desirable in a thick film printed circuit environment because the resin impregnated glass cloth can volatilize during the thick film sintering process.
A number of patents which disclose employing metallic substrates employ organic dielectric materials to coat the substrates. For example, U.S. Pat. No. 4,254,172 by Takahashi et al., issued Mar. 3, 1981; U.S. Pat. No. 4,679,122 by Belke, Jr. et al., issued July 7, 1987; and U.S. Pat. No. 4,677,252 by Takahashi et al., issued June 30, 1987, all disclose employing resins in the dielectric layer. U.S. Pat. No. 4,420,364 by Nukii et al., issued Dec. 13, 1983, discloses using a polyamic acid film or a polyamideimide film for a dielectric layer. As pointed out hereinbefore, a problem encountered with such organic materials is that they will not withstand the high temperatures encountered in thick film processes. U.S. Pat. No. 3,934,334 by Hanni, issued Jan. 27, 1976, discloses coating a metallic substrate with an electrostatic powder and curing at less than 500.degree. F. (260.degree. C.). These substrates, as described, are also used in connection with a non-high temperature procedure.
U.S. Pat. No. 4,365,168 by Chaput et al., issued Dec. 21, 1982, discloses a porcelain-coated metal (e.g. steel) substrate. The substrate can act as a ground plane. Printed circuitry can be applied to the porcelain-coated metal substrate by thick film techniques. However, problems caused by differential thermal expansion are not addressed by Chaput et al. The disclosed metal substrate material, steel, has a high coefficient of thermal expansion relative to, for example, alumina. This results in problems due to unequal thermal expansion when alumina leadless chip carriers are employed.
A number of patents disclose employing electrical connections to a metallic substrate. U.S. Pat. No. 3,296,099 by Dinella, issued Jan. 3, 1967, discloses a terminal region on a metallic substrate provided to make possible a common ground return for the printed circuit. The terminal region can be provided by masking the area, followed by the application of an epoxy resin or a polymeric thermoplastic resin. Alternatively, the terminal region can be provided by tapping, drilling or screwing a metal terminal through the resin insulation layer. However, the employment of a resin insulating layer precludes the use of high temperature thick film processes.
U.S. Pat. No. 3,202,591 by Curran, issued Aug. 24, 1965, discloses connecting electrical circuitry to a metal base by means of an aperture through the base so that the base may be grounded. The metal base is insulated by an electrolytically formed porous oxide layer on its surface. The aperture may be formed by masking, punching or otherwise forming a hole in the oxide layer. However, the electrolytically deposited oxide layer has porosity defects which must be treated with a silicon material in order to maintain the integrity of the insulating layer.
U.S. Pat. No. 4,328,614 by Schelhorn, issued May 11, 1982, discloses using a metal core as a common ground plane. However, the Schelhorn reference discloses the use of low carbon steel as the core material. As already pointed out, steel has a relatively high coefficient of thermal expansion.
In a paper entitled "Study of Thick Film on Stainless Steel Board", by Egawa et al., a method for producing a substrate by screen printing and sintering a glass paste on a steel core is disclosed. The problems encountered when steel cores are employed are discussed in the articles "Metal Core Materials for Thick Film Substrate Applications" by Schelhorn and "High Temperature Porcelain-Coated Copper-Clad Invar Substrates" by Hang et al. In particular, the steel core substrates are shown to have coefficients of thermal expansion which are considerably higher than 94%-96% pure alumina. Thermal cycling tests demonstrate that the expansion differential results in solder joint failure when alumina leadless chip carriers are soldered to porcelain-coated steel substrates.
In order to alleviate the problems inherent with the use of steel core substrates, Schelhorn and Hang et al. disclose the use of porcelain-coated copper-clad Invar (TM) as a circuit board support structure. The disclosed metal substrates have coefficients of thermal expansion which are relatively close to those of the porcelain coating at lower temperatures, e.g. from room temperature to about 200.degree. C. However, at higher temperatures the coefficient of thermal expansion for copperclad Invar (TM) substrates rises at a faster, non-linear rate than the coefficient for the porcelain coating, and reaches values similar to steel at about 800.degree. C. This non-linear behavior makes the design of a porcelain coating for a copper-clad Invar (TM) substrate difficult, because the coefficient of thermal expansion for the substrate is much higher than that for the coating at temperatures typically encountered during firing of the coating.
Therefore, it would be advantageous to have a circuit board which minimizes the problems associated with differential thermal expansion. In particular, it would be advantageous to have a substrate with a rate of thermal expansion which both approximates that for circuitry component materials such as alumina at operating temperatures, e.g. from about -55.degree. to about 125.degree. C., and approximates that for dielectric materials at higher temperatures, e.g. above 200.degree. C. Additionally, it would be advantageous to have an electrically conductive substrate so as to eliminate the need for an additional ground layer. In connection with the electrically conductive substrate, it would be advantageous to provide means for electrically connecting the circuitry to the substrate, while avoiding the need for drilling a hole through the insulating layer. Furthermore, it would be advantageous if the above-mentioned advantages could be employed on a circuit board prepared by a thick film process. Therefore, it would be advantageous if the electrically conductive substrate and the dielectric insulating layer could withstand high temperatures.